Libstdc++ has been using __STRICT_ANSI__ for years, e.g. to decide
whether or not std::is_integral<__int128> should be true.

gcc/ChangeLog:

	* doc/cpp.texi (Common Predefined Macros): Mention that
	__STRICT_ANSI__ is used by libstdc++ too.

OK for trunk?

I'd also like to tell users they're not allowed to undefine the
macro, would that be acceptable, or is that supposed to work?

I don't see how it's possible to make it work with libstdc++ if users
do stupid things like:

#include <type_traits>
#undef __STRICT_ANSI__
#include <iterator>

This causes the library to be in an inconsistent (broken) state,
because some templates are defined according to strict ISO C++ rules
and then other templates are not, but assume the earlier definitions
are consistent.

We frequently see users doing -std=c++11 -U__STRICT_ANSI__ which makes
me smash my head on the table. Just use -std=gnu++11 and stop being
dumb.